1. Field of the Invention
The present invention relates to a variable valve device for internal combustion engines, and more particularly, to techniques for optimizing valve opening/closing timings of intake and exhaust valves.
2. Description of the Related Art
In recent years, more and more internal combustion engines (engines) have come to be equipped with cam phase variable mechanisms as a variable valve device for varying valve opening/closing timings (cam phases). Also, techniques have been developed whereby the cam phase variable mechanisms are applied to an engine having each cylinder provided with a plurality of intake valves and the valve opening/closing timings of all intake valves as well as only some of the intake valves are varied (split) in accordance with the operating condition of the engine (Japanese Laid-open Patent Publication No. 2009-144521).
Where the valve opening/closing timings of only some of the intake valves associated with the individual cylinders can be varied in this manner, the multiple intake valves can be made to open continuously as a whole, whereby highly flexible valve control can be executed so that the overall valve open period of the intake valves may be prolonged, making it possible to improve the operational performance of the engine.
In general, it is preferable to set the closing timing of the intake valve to a most retarded position when the internal combustion engine is in a low speed and load state because pumping losses can be reduced by retarding the closing timing of the intake valve. Based on this view, according to the above-mentioned publication, when the internal combustion engine is in the low speed and load state as seen during start-up, idling, etc., one among a plurality of valves (one between two valves in this specification), whose opening/closing timing is variable, is controlled to the most retarded position.
However, the inventors' research discloses that, if one of the valves, whose opening/closing timing is variable, is controlled to the most retarded position as described above during an extremely low speed and load period after the warm-up of the internal combustion engine as in a hot idling state, this reduces pumping losses but incurs phenomena, such as unstable combustion and inefficient fuel consumption. It is undesirable to deteriorate combustion stability and fuel consumption during the extremely low speed and load period after the warm-up of the internal combustion engine. Considering that the idling is frequently conducted, a sufficient improvement cannot be seen in the operating performance of the internal combustion engine.